Effect of annealing treatment on optical properties and microstructural variation of $$\hbox {WO}_{3}/\hbox {Ag}/\hbox {WO}_{3}$$ multilayer nano-films

Structural and optical properties of $$\text {WO}_{3}/\text {Ag}/\text {WO}_{3}$$ nano-multilayer composites were investigated for heat mirror applications. $$\text {WO}_{3}/\text {Ag}/\text {WO}_{3}$$ thin films were fabricated through a physical vapour deposition method by using electron-beam evaporation at the vacuum chamber at 10 $$^{-5}$$ Torr. $$\text {WO}_{3}$$ nano-layer was fabricated at 40 nm. Annealing treatment was carried out at 100, 200, 300 and 400 $$^{\circ }$$ C for 1 h after the deposition of first layer of $$\text {WO}_{3}$$ on the glass. On $$\text {WO}_{3}$$ film, Ag nano-layers with 10, 12 or 14 nm thickness were deposited. Individual layers morphology was investigated using atomic force microscopy (AFM) and deduced that a smoother layer can be achieved after the annealing at 300 $$^{\circ }$$ C. Ellipsometry analysis was executed to determine both layers, Ag film thickness and inter-diffusion between the $$\text {WO}_{3}$$ –Ag– $$\text {WO}_{3}$$ layers. It was inferred that there was almost no interfering among the $$\text {WO}_{3}$$ – $$\text {WO}_{3 }$$ layers in the samples with 12 and 14 nm Ag thickness; while silver was deposited on the annealed $$\text {WO}_{3}$$ layer at 300 $$^{\circ }$$ C. UV–visible spectrophotometer showed that the annealing treatment of the first $$\text {WO}_{3}$$ layer enhanced the transparency of films in the visible region. The innovations of the present study have been based on the annealing of the films and finding an optimum thickness for the Ag film at 12–14 nm. Heat mirrors efficiency was assessed according to the principle of their optical behaviour and optimum performance obtained for 14 nm of Ag film, deposited on annealed tungsten oxide at 300 $$^{\circ }$$ C.